This disclosure relates generally to patient monitors and physiological sensors used for acquiring electrophysiological signals from a subject/patient. More particularly, the disclosure relates to monitoring the degradation of physiological sensors.
A prerequisite of patient care is that accurate and reliable measurements can be made from the patient to evaluate the patient's state. Since a patient monitor connected to a sensor may perform rather complex calculations based on the physiological signals acquired through the sensor and since the results obtained may depend on a variety of parameters related to the sensor, it is important that the sensor fulfills certain quality standards and is thus authorized to be used in the patient monitor for the measurement in question. The use of aged, damaged or low quality sensors may lead to inaccurate and/or unreliable results, which may in turn contribute to incorrect medical decisions and even risk patient safety.
In terms of patient safety, the use of non-authentic, unauthorized and/or counterfeited sensors is also to be prevented, since the cooperation of such sensors with the patient monitor is not tested and the sensors therefore involve the same risks as authentic but aged or low quality sensors.
It is therefore common practice to provide a sensor/monitor system with a detection mechanism that detects aged and/or unauthorized sensors, or with a mechanism that tends to improve the performance level of the sensor. The solutions may be classified into different categories according to the type of data stored in the sensor and according to the way in which data stored in sensor memory is employed. In one solution, the content of the sensor memory is used by a monitor algorithm to make the measurement more accurate. For this, the sensor memory may hold sensor parameters that are relevant to the measurement or provide different calibration coefficient sets for different two or more ranges of certain sensor parameters. The sensor parameters are typically variables that the patient monitor is incapable of measuring, such as LED wavelengths. The sensor memory may also hold operating parameters that prevent the use of the sensor outside a safe operating range. A further solution is to record other information related to the use of the sensor into the sensor memory, such as maximum usage time, expiration data, or warranty date of the sensor. This data may then be used to prevent the use of the sensor when the stored limit value is reached. Instead of measuring cumulated use time, the monitor may also measure the actual total amount of use. This may be carried out by counting the drive pulses transmitted to the sensor, for example.
Although current solutions are able to ensure a high quality sensor operation for the entire life of the sensor, the maximum life time of the sensor is typically determined on a statistical basis so that the risk of a sensor breakage or wear out during the pre-set and fixed life time is low enough in order not to risk patient safety. This also means that the safety margin, i.e. the time between the pre-set maximum life time and the real life time of the sensor is rather long for the majority of sensors. That is, most sensors are discarded even if there would be a considerable amount of life time left at the time of discard. The drawback is emphasized in environments where favorable conditions and good equipment care prolong the life of the sensor.
Consequently, the requirement of patient safety generally and inevitably translates into a shortened life time of the sensor, which means that the end user cannot get a maximal utility out of the sensor.